1. Field of the Invention
The present invention relates to an exposure device and an exposure method for exposing a photosensitive medium to light on the basis of image data including multiple pixels.
2. Description of Related Art
Photographic processors that adopt what is called a digital exposure system have been recently used extensively. Such a digital exposure system is capable of forming an image by exposing photographic paper to light modulated on the basis of digital image data. The adoption of the digital exposure system allows highly flexible image processing such as color correction, density correction, and sharpening processing and also rapid additional printing, thus providing high-quality printing with high color- and density-reproducibility and high resolution.
An example of the foregoing digital-exposure photographic processors is one that adopts a scanning exposure system in which a laser beam is used for scanning through a polygon mirror etc. to expose photographic paper to light. With the scan-exposure photographic processor, photographic paper is transferred in the direction perpendicular to a main scanning direction (in a sub scanning direction) by a specified feed during one scanning or every time the laser beam is scanned across the width of the photographic paper (in the main scanning direction). The exposure processing in the main scanning direction with the laser beam and the transfer of photographic paper in the sub scanning direction are repeated to form a latent image onto the photographic paper.
With the scan-exposure photographic processor, exposure is performed such that no clearance is formed between an exposure band based on image data containing multiple pixels corresponding to one main scanning (main scanning A) and an exposure band based on image data containing multiple pixels corresponding to main scanning (main scanning B) following the main scanning A. Here, in order to prevent the occurrence of banding due to feed variations of photographic paper and improve the resolution of an image, the region in the vicinity of the boundary between the exposure band based on the main scanning A and the exposure band based on the main scanning B is sometimes exposed by a main scanning (main scanning X) based on the same image data as that corresponding to the main scanning A (for example, refer to JP-A-10 (1998)-181086). In that case, the exposure band based on the main scanning X partially overlaps with the exposure bands based on the main scanning A and the main scanning B.
When the amount of deviation in the sub scanning direction on the photographic paper between the exposure band based on the main scanning A and the exposure band based on the main scanning X is half of the amount of deviation in the sub scanning direction between the exposure band based on the main scanning A and the exposure band based on the main scanning B, the resolution in the sub scanning direction of the image formed on the photographic paper is apparently twice as high as that in the main scanning direction. At that time, little clearance is formed between the dots corresponding to the multiple pixels contained in the image data of the main scanning A and the dots corresponding to the multiple pixels contained in the image data of the main scanning B on the photographic paper, thus preventing the distortion of image due to banding.
With the scan-exposure photographic processor, for the main scanning direction, a laser beam is emitted continuously. Thus, the fluctuation in light amount among the multiple pixels arranged in the main scanning direction (the fluctuation in light amount of a laser beam when the dots corresponding to the multiple pixels are formed) is relatively gentle. On the other hand, for the sub scanning direction, the laser beam is emitted intermittently (discontinuously), Thus, the fluctuation in light amount among the multiple pixels arranged in the sub scanning direction is sharp. As a result, the difference in density between two dots corresponding to two pixels neighboring in the sub scanning direction on the photographic paper is significantly larger than that between two dots corresponding to two pixels neighboring in the main scanning direction. Accordingly, when the case in which an image is viewed along the main scanning direction and the case in which the image is viewed along the sub scanning direction are compared, an image whose edge (the difference in density among dots) is enhanced in the sub scanning direction is produced, posing the problem of decreasing image quality.